Investigating the ring expansion reaction of pentaphenylborole and an azide

Article abstract of DOI:10.1039/c4cc04864d

The reaction between trimethylsilyl azide and pentaphenylborole was recently shown to produce the corresponding 1,2-azaborine. Investigating this transformation theoretically suggests that the reaction proceeds via coordination of the azide to the borole, rearrangement to a bicyclic species, and conversion to a kinetically favoured eight-membered BN₃C₄ heterocycle or expulsion of N₂ to furnish the thermodynamically favoured 1,2-azaborine. The eight-membered species was structurally characterized as a borole adduct and represents an unusual analogue of cyclooctatetraene. This journal is

Full text of DOI:10.1039/c4cc04864d

ChemComm
View Article Online
View Journal
COMMUNICATION
Investigating the ring expansion reaction of
pentaphenylborole and an azide†
Cite this:DOI: 10.1039/c4cc04864d
Shannon A. Couchman,a Trevor K. Thompson,b David J. D. Wilson,a
Jason L. Duttona and Caleb D. Martin*b
Received 26th June 2014,
Accepted 13th August 2014
DOI: 10.1039/c4cc04864d
www.rsc.org/chemcomm
The reaction between trimethylsilyl azide and pentaphenylborole was
recently shown to produce the corresponding 1,2-azaborine. Investi-
gating this transformation theoretically suggests that the reaction
proceeds via coordination of the azide to the borole, rearrangement
to a bicyclic species, and conversion to a kinetically favoured eight-
membered BN3C4 heterocycle or expulsion of N2 to furnish the
thermodynamically favoured 1,2-azaborine. The eight-membered
species was structurally characterized as a borole adduct and
represents an unusual analogue of cyclooctatetraene.
The remarkable isolation of the anti-aromatic pentaphenylborole A
in 1969 by Eisch marked a significant discovery in heterocyclic
chemistry (Scheme 1).1 This area was revitalized with the determina-
tion of the solid-state structure of A in 2008.2 The groups of Piers,
Braunschweig and others have recently reported great advances in
pursuing the reactivity of boroles and diversifying their synthesis to
other species incorporating the borole core.3–13 Boroles can display
redox reactivity within the heterocycle14–20 and Diels–Alder diene
reactivity with the butadiene backbone.21,22 In addition, they have
been shown to be powerful Lewis acids23–25 which is underscored
in the exposure of Eisch’s borole or Piers’ borole (perfluoropenta-
phenylborole, B) to an atmosphere of carbon monoxide. This
Scheme 1 (i) Reaction of boroles with CO, (ii) Piers’ borole with diphenyl-
acetylene, (iii) Braunschweig’s report on converting Eisch’s borole to the
1,2-azaborine (1) and the structure of kinetic product 2 reported in this study.
results in a rare example of a reversible main group CO adduct centre, aryl group migration to the b-carbon and finally ring
relying purely on s-donation (C).‡26
expansion to the six-membered product. The formation of the
The diverse reactivity of boroles is nicely illustrated by the borepin E occurs via a [4+2] cycloaddition between the butadiene
reaction of Piers’ borole with diphenylacetylene which produces backbone of the borole and the alkyne followed by a series of
two products: a six-membered ring with an exocyclic C–C double rearrangements. Interestingly, the less Lewis acidic borole A only
bond D and the seven-membered borepin heterocycle E.27 The participates in the Diels–Alder pathway.22
former occurs through coordination of the alkyne to the boron
Braunschweig et al. recently reported the reaction of trimethyl-
silyl (TMS) as well as organic azides with boroles that produced the
corresponding 1,2-azaborines (1) in high yield.28 This is a very
efficient route to 1,2-azaborines, which have garnered a lot of
attention to potentially serve as pharmaceuticals as well as in
electronic materials.29 In these studies, they observed a solution
colour change from the blue borole solution to red and eventually
to colourless. During the course of this reaction, N2 gas evolved as
the by-product. At the same time as this study appeared in the
literature we observed the same results in comparable yield.
a LaTrobe Institute for Molecular Science, Department of Chemistry,
La Trobe University, Melbourne, Victoria, 3806, Australia
b Department of Chemistry and Biochemistry, Baylor University, One Bear Place
#97348, Waco, TX, 76798, USA. E-mail: caleb_d_martin@baylor.edu
† Electronic supplementary information (ESI) available: Experimental details,
NMR spectra, X-ray crystallographic information, computational methods and
Cartesian coordinates of optimized geometries are provided. CCDC 1010370. For
ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/
c4cc04864d
This journal is ©The Royal Society of Chemistry 2014
Chem. Commun.

View Article Online
Communication
ChemComm
characterize 2. Redissolving the crystals in CD2Cl2 and acquiring a
1H NMR spectrum revealed signals for a species containing two
pentaphenylborole units to one TMS group (aryl multiplets d =
7.74–5.94 ppm and singlet at d = ꢀ0.22 ppm, respectively, integrat-
ing in a 50 : 9 ratio) consistent with the X-ray diffraction data. After
storing the NMR tube for 4 days, the solution colour changed from
red to the blue, the colour of free borole A. Obtaining 1H and 11
B
NMR spectra of the same sample in CD2Cl2 showed diagnostic
signals consistent with the quantitative conversion to the
1,2-azaborine product 1 (1H NMR: dTMS = ꢀ0.16 ppm and
11B{1H} NMR d = 40 ppm) and the free borole A (11B{1H}
NMR d = 65 ppm).2,28 This confirming that eight-membered
complex 2, is indeed converted to the thermodynamic product
1 over time and that adduct formation is reversible.
Fig. 1 Solid-state structure of 2. Hydrogen atoms are omitted for clarity
Theoretical studies were undertaken to establish the veracity
and thermal ellipsoids are drawn at the 50% probability level. Endocyclic
bond lengths (Å): B(1)–N(1) 1.477(3), N(1)–N(2) 1.340(3), N(2)–N(3) 1.284(3), of the proposed mechanism (Scheme 2) using B3LYP-D3BJ/
N(3)–C(4) 1.467(3), C(4)–C(3) 1.356(3), C(3)–C(2) 1.503(3), C(2)–C(1)
6-31+G(d) geometries and SCS-MP2/6-31+G(d)//B3LYP-D3BJ/
6-31+G(d) energies. In the computational studies a model system
1.353(3), C(1)–B(1) 1.574(4).
was employed that retained the full trimethylsilyl group on the azide
but with a simplified B-phenyl/C-methyl substituted borole system
0
Fascinated by this transformation, we investigated this chemistry for computational efficiency (notation of is used to represent the
further both computationally and experimentally allowing us to shed model system). All transition states were confirmed to connect the
light on the mechanism as azides can show a variety of reaction associated minima from extensive IRC calculations. The formation
pathways.30 These include Diels–Alder cycloaddition and coordination of the initial adduct of TMS–azide with the borole (30) was deter-
chemistry, both of which were observed in the aforementioned alkyne mined to be almost thermodynamically neutral (DG = ꢀ13 kJ molꢀ1
)
chemistry. In addition, azides can act as sources of nitrene fragments. with respect to the separated reagents. This adduct is connected to a
Through these studies, we suggest that the reaction proceeds via bicyclic intermediate 40 via a transition state (TS1), with a barrier of
coordination of the azide to form an adduct with the borole which 56 kJ molꢀ1. The bicyclic intermediate 40 is a key intermediate, due
then forms a fused bicyclic species. This species can then convert to to its presence in the reaction path to both products 10 and 20.
an eight-membered analogue of cyclooctatetraene via a reversible low
barrier pathway. Ultimately the bicyclic species expels N2 to form the a low transition barrier of 13 kJ molꢀ1 (TS2). The formation of
thermodynamic product, the 1,2-azaborine.
the cyclooctatetraene analogue from 40 is thermodynamically
Conversion of 40 to cyclooctatetraene derivative 50 occurs via
Conducting the same 1 : 1 reaction as the earlier report of TMS– favourable with a DG of ꢀ9 kJ molꢀ1. However, direct elimina-
azide and borole in toluene, we observed the same colour change to tion of N2 from 50 to generate 10 proceeds via a transition state
red after stirring the mixture for 1 min. Removing the solvent in with a barrier of approximately 175 kJ molꢀ1 (TS3), which
vacuo produced a red powder that displayed multiple TMS reso- is much too high for the experimentally observed rate and
nances by 1H NMR spectroscopy. Storing a saturated Et2O solution temperature of the reaction. If instead we consider 20, which
of the redissolved solids at ꢀ35 1C resulted in the formation of red is thermodynamically favoured and the borole adduct of 50
crystals. An X-ray diffraction study revealed a 1 : 2 azide to borole (DG = 85 kJ molꢀ1 from 50), the barrier for N2 elimination to
containing product (2; Fig. 1, Scheme 1). The three nitrogen atoms form 10 is also too high (104 kJ molꢀ1, TS4) for the observed
from the azide moiety were incorporated into one of the borole rings reaction. However, the calculated barrier from 40 to 10 is readily
generating an eight-membered BN3C4 ring. The g-nitrogen atom surmountable at room temperature (51 kJ molꢀ1). This reaction
formerly from the azide was found to be coordinated to a second involving elimination of N2 to form 10 is highly favourable (DG =
equivalent of borole. Interestingly, this new eight-membered hetero- ꢀ359 kJ molꢀ1 from 40), making the final transformation
cycle represents an analogue of cyclooctatetraene.31 This species is irreversible. The ability to isolate 2 if the reaction is stopped
only the second report of any sort of a B,N-analogue of cycloocta- quickly can be rationalised by 2 lying in an energy well provided
tetraene. The other example was recently reported by Bettinger, a B4N4 by favourable adduct formation with the second equivalent of
heterocycle featuring alternating boron and nitrogen atoms.32,33 borole. The formation of 2 at the start of reaction is due to the
However, complex 2 is the first report of a hybrid inorganic/organic lower barrier at TS2 as compared to TS5. The complete conver-
species. Much like its carbonaceous relative and the inorganic B4N4 sion to 1 in the initial reaction and the observation that 2
system,31,32 2 avoids an anti-aromatic configuration by adopting a converts to 1 if isolated and subsequently redissolved can be
non-planar geometry with alternating single and double bonds in a explained by modest barriers for the reverse reaction over TS2
pseudo-boat conformation.
(23 kJ molꢀ1) leading back to 30. No transition state could be
Compound 2 could be isolated as crystals in 8% yield when found for the formation of adduct 20 from 50, but the barrier is
altering the stoichiometry to a 1 : 2 ratio of TMS–azide to borole not likely to be large, especially given the ease with which the
and working up the reaction after 1 minute allowing us to reverse reaction is observed to occur.
Chem. Commun.
This journal is ©The Royal Society of Chemistry 2014

View Article Online
ChemComm
Communication
Scheme 2 Calculated pathway leading to compounds 50 and 10. The energy under the compounds represents the relative energy (DG) of that
‡
0
compound with respect to A0. The energies over the arrows represent DG for the transformation in the indicated direction. Note: the symbol signifies
methyl groups have been used to differentiate the model system from the all phenyl system in experiments.
10 A. Y. Houghton, V. A. Karttunen, W. E. Piers and H. M. Tuononen,
Chem. Commun., 2014, 50, 1295–1298.
11 H. Braunschweig, I. Krummenacher and J. Wahler, in Adv. Organo-
met. Chem., ed. A. F. Hill and M. J. Fink, Academic Press, San Diego,
CA, 2013, ch. 1, vol. 61, pp. 1–53.
12 A. Steffen, R. M. Ward, W. D. Jones and T. B. Marder, Coord. Chem.
Rev., 2010, 254, 1950–1976.
13 S. Biswas, I. M. Oppel and H. F. Bettinger, Inorg. Chem., 2010, 49,
4499–4506.
This report demonstrates that an azide reacts with a borole
initially through coordination of the nitrogen atom to the borole
rather than via a Diels–Alder or a nitrene route. The coordination
is followed by a bicyclic ring formation that can proceed via two
pathways, a reversible pathway to an eight-membered heterocycle,
which could be experimentally isolated, or an irreversible pathway
to yield the 1,2-azaborine product. The kinetic product represents
the first isolated hybrid inorganic/organic analogue of cycloocta-
tetraene. The described synthesis of 1,2-azaborines from boroles
is restricted to peraryl derivatives. However, highly substituted
1,2-azaborines are challenging to prepare and this synthesis could
provide a facile route to such species. In summary, we herein
further demonstrate the diverse, as well as peculiar, reactivity of
boroles and anticipate that this study will provide insight on new
avenues in borole chemistry.
14 H. Braunschweig, C. W. Chiu, J. Wahler, K. Radacki and T. Kupfer,
Chem. – Eur. J., 2010, 16, 12229–12233.
¨
15 J. Bauer, H. Braunschweig, C. Horl, K. Radacki and J. Wahler, Chem.
– Eur. J., 2013, 19, 13396–13401.
16 H. Braunschweig, C. W. Chiu, D. Gamon, M. Kaupp,
I. Krummenacher, T. Kupfer, R. Mu¨ller and K. Radacki, Chem. –
Eur. J., 2012, 18, 11732–11746.
¨
17 H. Braunschweig, K. Kraft, S. Ostreicher, K. Radacki and F. Seeler,
Chem. – Eur. J., 2010, 16, 10635–10637.
18 H. Braunschweig, A. Damme, D. Gamon, H. Kelch,
I. Krummenacher, T. Kupfer and K. Radacki, Chem. – Eur. J., 2012,
18, 8430–8436.
19 H. Braunschweig, V. Dyakonov, J. O. C. Jimenez-Halla, K. Kraft,
I. Krummenacher, K. Radacki, A. Sperlich and J. Wahler, Angew.
Chem., Int. Ed., 2012, 51, 2977–2980.
20 C. W. So, D. Watanabe, A. Wakamiya and S. Yamaguchi, Organo-
metallics, 2008, 27, 3496–3501.
21 J. J. Eisch and J. E. Galle, J. Am. Chem. Soc., 1975, 97, 4436–4437.
22 J. J. Eisch, J. E. Galle, B. Shafii and A. L. Rheingold, Organometallics,
1990, 9, 2342–2349.
This work was supported by the Welch Foundation (Grant No.
AA-1846), Baylor University, The La Trobe Institute for Molecular
Science, NCI-NF and VPAC for computing resources, and the
Australian Research Council (DE130100186). We thank John
Wood and his research group for helpful discussions and Kevin
Klausmeyer for assistance with X-ray crystallographic studies.
23 H. Braunschweig, C. W. Chiu, T. Kupfer and K. Radacki, Inorg.
Chem., 2011, 50, 4247–4249.
Notes and references
¨
24 H. Braunschweig, C. W. Chiu, D. Gamon, K. Gruß, C. Horl, T. Kupfer,
K. Radacki and J. Wahler, Eur. J. Inorg. Chem., 2013, 1525–1530.
25 K. Ansorg, H. Braunschweig, C. W. Chiu, B. Engels, D. Gamon,
M. Hu¨gel, T. Kupfer and K. Radacki, Angew. Chem., Int. Ed., 2011, 50,
2833–2836.
26 A. Fukazawa, J. L. Dutton, C. Fan, L. G. Mercier, A. Y. Houghton,
Q. Wu, W. E. Piers and M. Parvez, Chem. Sci., 2012, 3, 1814–1818.
27 C. Fan, W. E. Piers, M. Parvez and R. McDonald, Organometallics,
2010, 29, 5132–5139.
‡ With Eisch’s borole, at low temperatures the adduct is stable, but at
room temperature it ring expands to a six-membered ring.
1 J. J. Eisch, N. H. Hota and S. Kozima, J. Am. Chem. Soc., 1969, 91,
4575–4577.
2 H. Braunschweig, I. Fernandez, G. Frenking and T. Kupfer, Angew.
Chem., Int. Ed., 2008, 47, 1951–1954.
3 A. Y. Houghton, V. A. Karttunen, C. Fan, W. E. Piers and
H. M. Tuononen, J. Am. Chem. Soc., 2013, 135, 941–947.
4 H. Braunschweig and T. Kupfer, Chem. Commun., 2011, 47,
10903–10914.
5 T. Araki, A. Fukazawa and S. Yamaguchi, Angew. Chem., Int. Ed.,
2012, 51, 5484–5487.
¨
¨
28 H. Braunschweig, C. Horl, L. Mailander, K. Radacki and J. Wahler,
Chem. – Eur. J., 2014, 20, 9858–9861.
29 P. G. Campbell, A. J. V. Marwitz and S. Y. Liu, Angew. Chem., Int. Ed.,
2012, 51, 6074–6092.
30 E. F. V. Scriven and K. Turnbull, Chem. Rev., 1988, 88, 298–368.
6 H. Braunschweig, C. W. Chiu, K. Radacki and P. Brenner, Chem. 31 J. Bordner, R. G. Parker and R. H. Stanford, Jr., Acta Crystallogr., Sect.
Commun., 2010, 46, 916–918. B: Struct. Crystallogr. Cryst. Chem., 1972, 28, 1069–1075.
7 H. Braunschweig, A. Damme, C. Horl, T. Kupfer and J. Wahler, 32 M. Muller, C. Maichle-Mossmer and H. F. Bettinger, Angew. Chem.,
¨
¨
¨
Organometallics, 2013, 32, 6800–6803.
Int. Ed., 2014, 53, DOI: 10.1002/anie.201403213.
¨
¨
8 A. Iida, A. Sokioka and S. Yamaguchi, Chem. Sci., 2012, 3, 1461.
9 J. F. Araneda, B. Neue, W. E. Piers and M. Parvez, Angew. Chem.,
Int. Ed., 2012, 51, 8546–8550.
33 S. Biswas, M. Mu¨ller, C. Tonshoff, K. Eichele, C. Maichle-Mossmer,
A. Ruff, B. Speiser and H. F. Bettinger, Eur. J. Inorg. Chem., 2012,
4634–4639.
This journal is ©The Royal Society of Chemistry 2014
Chem. Commun.